Typically, a crankcase ventilation passage opens into the crankcase of an engine for recycling the blowby gas, which has passed through the gap between the pistons and cylinders, to the intake system, and to control the pressure pulsation due to the reciprocating motion of the pistons (see Japanese patent laid-open publication No. 61-135914).
The crankcase is filled with lubrication oil mist, and a certain amount of the oil mist inevitably enters the ventilation passage. However, an excessive introduction of oil into the blowby gas is not desirable because it adversely affects the quality of the exhaust gas and contributes to an increase in oil consumption. To avoid such an inconvenience, it has been proposed, for instance, in Japanese patent laid open publication No. 61-135914 to provide an oil separation chamber between the two cylinder banks of a V-engine.
However, to achieve an adequate capability to separate oil, the oil separation chamber is required to have a certain volume, and the provision of such an oil separation chamber in the engine results in an increase in the number of components parts, and the complication and size increase of the overall structure.
The blowby gas removed from the crankcase must be replaced by fresh air from the atmosphere. The pressure pulsation in the crankcase can be transmitted from a fresh air passage for introducing fresh air into the crankcase. Also, it is possible for the blowby gas to flow backward under special circumstances. To address these problems, it is therefore desirable to provide a relatively large passage or chamber for fresh air for both effective noise muffling and oil separation. However, it prevents a compact design of the engine.
The blowby gas is typically passed through a passage which is adapted to remove oil mist therefrom, and is then forwarded to the downstream of a throttle valve so that hydrocarbon that may be contained in the blowby gas may be recycled to the engine intake to improve fuel efficiency and reduce engine emissions. Therefore, a passage must be defined between the crankcase typically provided in a lower part of the engine and the intake system which is typically provided in an upper part of the engine, and the need for such a passage tends to complicate the structure of the engine. Typically, rubber hoses are used for conducting blowby gas from the crankcase to the intake system.